In order to satisfy continually increasing wireless data traffic demands in a wireless communication system, solutions for supporting a higher data transmission rate have been studied. One of the solutions is to use a beamforming-based base station which utilizes a wide frequency band in a millimeter wave (mmWave) band, thereby significantly enhancing the capacity of a cellular system.
Meanwhile, in order to transmit a multitude of informations to a single user or a multi-user, a MIMO (Multiple Input Multiple Output) system, which is considered in the existing standards such as LTE (Long Term Evolution)-Advanced, has a plurality of digital paths or RF (Radio Frequency) chains. When MIMO communication is performed using such digital paths, the performance gains such as a diversity gain and a multiplexing gain may be obtained. However, when the number of digital paths is increased to obtain a greater gain, problems such as synchronization between digital paths, costs, and operation complexity may be generated.
In a millimeter wave band system, the disadvantage of path attenuation may be offset through the beamforming gain by using a large number of physical antennas. However, the digital beamforming scheme in the existing MIMO system needs one RF chain for one physical antenna, and thus a large number of RF chains are needed. This causes problems such as costs and operation complexity. Hence, in order to perform efficient communication in the millimeter wave band, a hybrid beamforming system, which simultaneously uses a digital beamforming and an analog beamforming, may be considered. The analog beamforming may connect a plurality of physical antennas in one RF chain as an array and may form a narrow beam using a phase shifter. Compared to the digital beamforming, the analog beamforming is poor in aspect of the sharpness of the beam and flexibility of direction adjustment, but the analog beamforming does not increase the number of digital paths, thereby having low implementation costs and low complexity. In order to efficiently obtain a high communication capacity in a millimeter wave band, a hybrid beamforming system, in which advantages and disadvantages of the digital beamforming and the analog beamforming are appropriately combined, may be considered.